Circular hollow concrete poles are well known in applications ranging up to 30 meters in length. A process for constructing these hollow poles involves a spinning process where centrifugal force is used to spread a wet concrete mixture over the inner surface of a horizontally arranged mould. Concrete is fed into the slowly revolving mould which is then spun, squeezing out surplus water and evenly spreading compacted concrete over the inner surface.
While this method of forming hollow concrete poles has the benefit that the concrete wall of the pole is both dense and strong as a result of the centrifugal spinning action there are a number of serious drawbacks to this process. The first disadvantage is the lack of uniformity of wall thickness of the pole which can vary from one pole to another. The spinning process first involves the concrete being placed in the mould before it has been fully assembled. It is then left to an operator to decide precisely how much concrete is placed in any particular part of the mould which can lead to a degree of variability.
Where the pole is tapered, which is required in many standard applications such as a telecommunications or power transmission pole, when the mould is spun the concrete will not always relocate around and along the mould uniformly. In these circumstances, the concrete will as a result of the centrifugal force be distributed along the tapered profile to the place of least resistance at the thicker end of the mould. This results in a pole that will not have a uniform bending capacity because of the variation in wall thickness. Where the spinning process is used in combination with a prestressing process, the variation in thickness is likely to also cause the pre-stressed pole to distort immediately once it is removed from the mould.
A further and significant problem with a spun pole is the advent of a thick layer of laitance on the inner surface of the finished product. This layer is highly absorbent and can cause ground water to move up the inside surface of the pole and into the concrete. If this water contains any salt, the pH of the concrete will be reduced, thereby causing corrosion of any reinforcement of the pole.